After Republican President Donald Trump held high-level talks with Venezuelan opposition leader Juan Guaido, Venezuela's socialist government quietly moved a few dozen U.S. company executives, including executives of Chevron Corp. and Exxon Mobil Corp., to more difficult jails where they could spend longer than ever in solitary confinement, a prison expert said.

Sixteen employees of the U.S. subsidiaries of Venezuela's national oil company PDVSA were kept under constant guard and moved to different "cell blocks" at the Ramo Verde prison south of Caracas after they were permitted to leave the crude exporting refinery, which was converted into a military base, according to U.S. company executives and the U.S. Embassy in Caracas.

The officials said they were scared for their safety and had requested help to be released, but were denied.

"I don't know when I'll see them again," said Natalia Rueda, who worked for Chevron's subsidiary for eight years until last month at the company's unit in Zulia state in Venezuela's western plains. "Some of them have been here for so long that I have felt like they are like family. The shelter, what I eat, the running water, the showers, have changed. Now it is just pitiful."

Government officials did not respond to requests for comment on the employees' situation. But they defended the detentions, saying the company employees had helped to organize anti-government protests.

"We feel responsibility for our country and our people and will not be cowed by these unscrupulous, desperate, machiavellian foreign capital groups that aim to sow discontent and civil war," Chief of Staff Jorge Rodriguez said in a statement.

In the statement, he accused Chevron's executives of cooperating with the opposition and the United States "by taking part in illegal demonstrations." The PDVSA officials said the Houston-based company had "failed to live up to obligations to respect the Venezuelan laws," according to the statement.

The opposition and U.S. officials, including Secretary of State Mike Pompeo, routinely blame the PDVSA employees for intimidating protesters in the months before President Nicolas Maduro's disputed re-election in May.

Exxon Mobil also took part in protests that "encouraged disturbances," according to a statement by the Venezuelan Oil Ministry on Tuesday.

The U.S. State Department said it had been contacted by Exxon's employees but declined to comment.

The violence between protesters and security forces across Venezuela swelled into the third week of protests on Wednesday, and about 26 people have been killed as the nation of 30 million is enduring one of the worst crises in its modern history.

The Maduro government's measures followed the detention of Guaido on Thursday, who has rejected Maduro's claim to be the legitimate leader following the Supreme Court's removal of a congressional power amid rising opposition to Maduro's increasingly authoritarian government.

Both Chevron and Exxon said they recognize Guaido's "right to assume the presidency to ensure the restoration of constitutional order in Venezuela."

But both Chevron and Exxon also had substantial interests in Venezuela.

For example, in 2014, Chevron said it had agreed to sell its interest in a crude refinery to Brazilian firm Petrobras for about $2.7 billion, according to the Houston Chronicle.

Petrobras did not respond to a request for comment on the refinery's closing.

But it has made numerous investments in Venezuela, including refinery complex Naftolchimos just outside the capital and an onshore oil field.

"We are committed to our relationship and cooperation with the Venezuelan people," a Petrobras spokesman said when asked about Chevron's refinery's closing.

In a statement to Venezuela's state media agency, Exxon said that it was "confident" it would "fully resolve its participation in the joint projects."

Concern over the future of the superpetrochemical facility began circulating after an accident at the Naftolchimos refinery last week that Venezuela's state oil company PDVSA said damaged about 900 tons of petrochemical products in mid-January.

A BP investigation published in 2013 found that Naftolchimos' ethylene production had suffered "regular disruption" over the previous three years that resulted in repeated delays in shipments, including to France's Arkema, Shell and Shell Guyana Petroleum.